In machine tools, it is sometimes desirable and necessary to clamp a workpiece to a machine table by means of a magnetic clamp device. For example, a grinding machine chuck includes a surface to mount the workpiece and may have imbedded therein an electromagnet for securing the workpiece thereto by virtue of magnetic flux. Since there is no mechanical lock between the workpiece and the chuck surface, it is possible that in the event of a magnetic clamp failure or merely inadequate clamping force, a workpiece may tip or slip on its locating surface. If such movement of a workpiece occurs, machining accuracies are destroyed resulting in a scrap workpiece.
It is known in the electric servomotor art used for machine slide axis speeds that the motors can momentarily exceed the rate of continuous thrust thereby resulting in a jerky movement and exceleration. This thrust of the machine slide during the machining process may be sufficient to overcome the clamping force exerted on the workpiece by the magnetic clamp. Therefore, the workpiece will change its position relative to the cutting tool.
Prior art devices have attempted to compensate for a magnetic clamp failure and subsequent part movement by providing interlock clamps which when actuated, mechanically secure the workpiece. The difficulties inherent in these devices are that the mechanisms themselves are not readily adapted to a wide variety of workpieces and have to be rearranged for different applications. Further, additional expense and design effort is often involved in providing the necessary failsafe features.
Applicant believes he has solved the difficulty inherent in the prior art devices by the disclosed apparatus which may be readily adapted to existing electric machine slide servomotors. Applicant discloses apparatus which reduces the probability of magnetic clamp failure on a machine tool and may be adapted with relative ease and moderately low cost. Applicant discloses further apparatus which is operative to inhibit machine slide motion under certain conditions which may lead to a magnetic clamp failure.